The boiler heats the water and the heated water is pumped around the system into the radiators.
Radiators are more efficient when they are full of water. The heated water transfers the heat to the radiators and into the house.
They have bleed valves so that air may be allowed to escape - you open the bleed valve and listen to the hissing air - when water comes out you close the bleed valve - best to have a small cloth handy to wipe up any escaped water.
They have an inlet valve on one side (your TRVs) and a lock-shield valve on the other side. Balancing radiators is the action of progressively closing - or opening - the lock-shield valves to make the heat loss similar in all radiators - if not done then the first few rads on the system can sometimes "steal" all the heat leaving the rads at the back end of the flow too cold.
Peccavi
January 2011